In ink jet printing systems a printed image is made up of individual droplets of ink generated at a nozzle and propelled towards a substrate. There are two principal systems: drop on demand where ink droplets for printing are generated as and when required; and continuous ink jet printing in which droplets are continuously produced and only selected ones are steered towards the substrate, the others being recirculated back to an ink supply.
Continuous ink jet printers supply pressurized ink to a print head drop generator where a continuous stream of ink emanating from a nozzle is broken up into individual regular drops by, for example, an oscillating piezoelectric element. The drops are steered past a charge electrode where they are selectively and separately given a predetermined charge before passing through a transverse electric field provided across a pair of deflection plates, including a high voltage plate and a zero/negative voltage plate. Each charged drop is deflected by the field by an amount that is dependent on its charge magnitude before impinging on the substrate whereas the uncharged drops proceed without deflection and are collected at a gutter from where they are recirculated to the ink supply for reuse. The charged drops bypass the gutter and hit the substrate at a position determined by the charge on the drop and a position of the substrate relative to the print head. Typically the substrate is moved relative to the print head in one direction and the drops are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head between drops arriving means that a line of drops would otherwise not quite extend perpendicularly to the direction of movement of the substrate).
In continuous ink jet printing a character is printed from a matrix including a regular array of potential drop positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line including a plurality of potential drop positions (e.g., seven) determined by the charge applied to the drops. Thus each usable drop is charged according to its intended position in the stroke. If a particular drop is not to be used then the drop is not charged and it is captured at the gutter for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix.
As the ink is ejected from the print head assembly, it is deposited on the substrate. However, ink in the form of ink mist or droplets may instead land on a surface proximate a flight path envelope of the ink droplets, such as an outer surface of the gutter. Over time this ink may accumulate and eventually protrude into the flight path envelope where it becomes an obstacle to the ink droplets closest to the accumulated ink. An ink-droplet that encounters the accumulated ink may be blocked and/or deflected from its intended flight path, and thus the intended print is not achieved. This can eventually lead to a shutdown of the printing process.